A disclosed generic shaping tool (publication BLECH [SHEET METAL], No. 9/1963, page 575, FIG. 7) consists essentially of a water tank, a drawing ring and drawing punch, and an actuatable blank holder.
The water tank is designed with side walls and a bottom which as lower tool may be filled with a liquid active medium and thus forms the drawing die. The drawing ring is plate-shaped in design and is mounted in the upper area of the water tank. It may be sealed by its ring area from a subjacent active medium volume. The edge of the ring aperture of the drawing ring forms a drawing edge for the workpiece.
The drawing punch is designed as to be movable by means of an adjustable driver rod so that it may be displaced to the area of the active medium, with the sheet metal blank interposed, through the ring aperture of the drawing ring. The movable blank holder serves as mounting support of the edges of the sheet metal blank introduced, the holding pressure selected being such that the possibility is provided of sliding of the edges of a sheet metal blank when a load is applied to the sheet metal blank by the drawing punch.
Specifically, the ring area of the drawing ring in this instance fits tightly against the upper edge of the side walls of the water tank. In addition, the blank holder presses the edge area of the sheet metal blank against a seal engaged in the drawing ring and accordingly seals the system containing the active medium off from the exterior. In the drawing process the drawing punch extends into the active medium, generally a water-oil emulsion, with the sheet metal blank interposed, as a result of which this medium is displaced by way of an adjustable throttle. The reaction pressure obtained as a function of the throttle position causes the sheet metal blank to be pressed against the drawing punch, as a result of which a workpiece of suitable shape may be produced.
A single shaping tool may be used to produce different workpieces with a water tank of predetermined size by replacement and use of different drawing rings each with a different ring aperture and of adjusted drawing stamp size. The following problems may arise in this instance.
With relatively large workpieces the ring area of the drawing ring rests on the side walls of the water tank so that the ring extends only a short distance above the side walls of the water tank. In the shaping process and with build-up of pressure in the active medium this medium exerts pressure upward over this short distance, but because of the small width of the area involved this pressure can be controlled by simple means and does not cause appreciable deformation of the drawing ring. The situation is different in production of a smaller workpiece in which use is made of a drawing ring with relatively smaller ring aperture, the water tank rigidly integrated into the shaping tool being used in this instance as well. The area of the drawing ring in this situation extends a considerable distance above the supports on the side walls of the water tank above the active medium toward the center of the water tank. During the shaping process the pressure in the active medium consequently exerts considerable force on the extension vertically upward on the drawing ring. This can result both in permanent deformation of the drawing ring during the shaping process and/or in elastic deformation of the drawing ring during the forming process as a sort of “breathing,” so that the quality of output and reproducibility may be adversely affected and uncontrolled in workpiece production. Such deformation could perhaps be prevented by higher stability of such a drawing ring, but the thickness of the drawing ring plate would have to be increased in comparison to drawing rings having smaller ring apertures. This would result in unfavorably higher production costs and higher weights associated with difficulties in drawing ring replacement, to which would have to be added inconvenient adjustments of the blank holder positions and drawing punch paths to drawing rings of different sizes.